besides the 3 ft of additional length and extra 800 or 900lbs what are the differences between these two boats?

a lot of money.. LOL

less speed in the 42 with same power

i believe the cockpit is smaller in the 42 also, call phil he will give you the 411

I'd probably buy the 39 Unlimited over the 42-X now days. That 39 Unlimited w/700's & NXT's, is the perfect "all around" Cigarette to me.

As Wild said, the main differences, are mostly the obvious ones. A bit less speed for the 42-X with identical setups, slightly more wetted surface to slice waves for the 42-X, and more boat to store & transport with the X.

It's going to be personal preference in the end. You can't go wrong either way.